Power converters have been widely applied in electronic devices. A power converter system generally includes multiple converter modules connected in parallel between a power input and a power output. FIG. 1 shows a converter module 100 of a conventional power converter system, which includes a controller 102 and an output stage 104 driven by the controller 102. The output stage 104 has two power switches 120 and 122 serially connected between a power input Vin and a ground node GND, and two signals Ug and Lg provided by the controller 102 switch the power switches 120 and 122 respectively, thereby modulating a phase current IL flowing through an inductor L in the output stage 104 to a power output Vout. Each of the other converter modules of the power converter system has the same configuration as that shown in FIG. 1. All phase currents of the power converter system are combined into an output current Iout to charge an output capacitor C to thereby generate an output voltage Vout. The controller 102 is a semiconductor chip, which has a pin FB to receive a feedback signal VFB representative of the output voltage Vout, an error amplifier 106 to compare the feedback signal VFB with a reference signal Vref to generate an error signal VEA, and a comparator 108 to compare the error signal VEA with a ramp signal Ramp to generate the signals Ug and Lg. The components in the converter modules may not be matched to each other between the converter modules, and thereby cause current imbalance between the converter modules. Therefore, a current balance mechanism is needed to balance between the phase currents.
For current balance between the converter modules, the controller 102 has a pin SENSE to receive a current sense signal Vs representative of the phase current IL of the associated converter module 100. In the controller 102, the current sense signal Vs is amplified by an amplifier 110 and then biased by a bias voltage source 112 to generate a voltage VI. The bias voltage source 112 prevents the converter nodule 100 from switching between a balance state and a current-adjusting mode. An operational amplifier 114 is configured as a voltage follower to apply the voltage VI to a pin SHARE of the controller 102. The pin SHARE of each converter module of the power converter system is connected to a share bus 124, and therefore the voltage VI_max on the pin SHARE of each converter module will be the maximum of the voltages VI of all the converter modules. In the controller 102, a diode 116 is connected between the output of the operational amplifier 114 and the pin SHARE to prevent reverse current flowing from the pin SHARE to the output of the operational amplifier 114, and an amplifier 118 amplifies the difference between the voltage VI and the maximum voltage VI_max to generate an output signal injecting into the non-inverting input of the comparator 108 for modulation of the phase current IL. The voltage VI represents the phase current IL of the associated converter module 100 while the maximum voltage VI_max represents the maximum one IL_max among all phase currents IL of the power converter system. The comparison between the voltages VI and VI_max can be regarded as the comparison between the phase currents IL and IL_max, and therefore the output of the comparator 118 will adjust the phase current IL of the associated converter module 100 toward the maximum phase current IL_max of the converter modules. For more detailed description of this current balance mechanism, readers are referred to U.S. Pat. No. 6,642,631 to Clavette. However, this current balance mechanism uses a complicated circuit to obtain the information of the phase currents IL and IL_max, and the bias voltage source 112 and the diode 116 will also affect the current balance accuracy.
Therefore, it is desired a simple circuit and method for improving current balance accuracy of a power converter system.